Digital plotter in which plotting speed is optimized in terms of a curve-fitting technique

ABSTRACT

Plotting of digital data is accomplished by optimizing plotting speed in terms of a curve-fitting technique. Segments of a curve whose x and y end points are specified, are defined in terms of a set of parametric equations represented by third order polynomials with distance as the independent variable. The curvature of these segments is then developed. Optimum plotting speed is then determined considering the requisite degree of curvature and the physical limitations of the plotter such as arm and pen mass and motor torque. This optimum plotting speed determines the rate of generation of the parametric equations in x and y and drives the system through a hybrid servosystem for the independent variable distance. Thus, the plotter is driven to the desired data points with a geometry independent of velocity.

United States Patent [72] Inventors William Cliiford,.]r. 3,293,651 12/1966 Gerber et al 235/l51 X Middletown; 3,328,655 6/1967 Tripp 318/20.l30 X John G. Demko, New Shrewsbury; William 3,449,554 6/ 1969 Kelling 235/15 l .1 1 K. Kindle, West Long Branch, all of NJ. 3,486,012 12/1969 Burnett et al 235/151 X 1 p 789,632 OTHER REFERENCES [22] Flled 1968 Control Engineering- Jan. l96l New Continuous Path [45] Patented Sept. 28, 1971 H System Uses ADA lnterpolator Henegarpg. 71- 76 (Copy [73] Assignee Electronic Associates, Inc.

, available in group 236-c1ass 235/150.31)

Long Branch, NJ. Continuation-impart of application Ser. No. Primary Examiner-MalcolmA. Morrison 725,418, Apr. 30, 1968, now abandoned. Assistant Examiner-Joseph F. Ruggiero AttorneysEdward A. Petko and Robert M. Skolnik [54] DIGITAL PLOT'IER IN WHICH PLOTTING SPEED ls OPTIMIZED [N TERMS OF A CURVEFFHTING A B STRACT: Plottmg of digital data ls accomplished by op- TECHNIQUE tlmizmg plotting speed in terms of a curvefitting techn que. 10 Claims 16 Drawing Figs. Segmgrgs odf a cuzve whosfe x and y cfmd points are specified, are e me m erms o a set 0 parametric equations [52] US. Cl ..235/l51.ll, represented by third Order polynomials with distance as the 318/573 340/1725 dependent variable. The curvature of these segments is then [51] Ill. Cl ..Go6l) 15/46 d l d O i p g speed is then determined [50] Field of Search 235/151, sidering the requisite degree of curvature and the physical l; 346/29 34; 340/1725; limitations of the plotter such as arm and pen mass and motor BRO-10530108120130 torque. This optimum plotting speed determines the rate of generation of the parametric equations in x and y and drives [56] References cued the system through a hybrid servosystem for the independent UNITED STATES PATENTS variable distance. Thus, the plotter is driven to the desired 3,227,864 1 1966 Kadoguchi 235/15 1.1 1 data points with a geometry independent of velocity.

MAG. TAPE DATA OR BUF- COMPUTER FERS 4 u 4a DDA oovm LOAD couuren CONTROL K ls a |7 SERVO DOA ADDER I i COMPARE PATENTEUSEP28I97| 3,609,319

sum 01 ur 15 MAX v MAX UMAX

FIGURE 2 FIGURE l PAIENIEUSEPE8IQYI 3,609,319

SHEET I'JII [1F 15 MULTIPLY ALL XSIY COORDINATES BY ZJSCALE FACTOR END OF PREVIOUS LINE MOVE CARRIAGE WITH PEN UP TO COORDINATES OF FIRST DATA POINT INITIALIZE CONSTANTS FIGURE 5 PATENTEDSEP28l9?! 3,609,319

SHEET as HF 15 IS THIS THE FIRST POINT IS THIS THE NEXT TO LAST POINT IS THIS A YES CLOSED CURVE COMPUTE DEL x| ,DEL YI, coMPuTE DEL x2, SIT-LAST" AND DEL x2, DEL Y2, A SIN I, DEL Y2, A SIN I, "I FLAG" INDICATORS Ac0sI,AsIN2,Acos2 Ac0s|,AsIN2,

A cos 2 COMPUTE COEFFICIENTS AX,AY,BX,BY,CX,CY" AND SEGMENT LENGTH z Do ANY OF THE COEFFICIENTS EXCEED THE MAXIMUM vALuE ALLowED BY THE HARDWARE SCALING RECOMPUTE COEFFICIENTS AND SEGMENT LENGTH sET VELOCITY ADJUST FLAG "Kx" FIGURE 6 PATENTEDSEP28IHTI 3,609,319

SHEET us or 15 COMPUTE "x'ouT" VALUES (IE. INTEGERIZE COEFFICIENTS AND SEGMENT LENGTH) IS THIS A PEN DOWN SEGMENT RESET CONTROL WORD"! OUT I" FOR PEN uP RESET 1 FLIP" so THAT NEXT SEGMENT WILL BE PEN UP RESET "1 FLIP" so THAT NEXT SEGMENT WILL BE PEN uP TRANSFER INTEGERIZED COEFFlClENTS CONTROL WORDS,

SEGMENT LENGTH AND VELOCITY TO COMMON BUFFER STORAGE (NOTE VELOCITY= O FIGURE 7 PATENTEU8EP28I9T| 3 509 319 SHEET 07 0F 15 22\ HAS THE FIRST HALF OF THE COMMON BUFFER STORAG AREA BEEN COMPLETELY FILLED YES WRITE CONTENTS OF FIRST HALF OF COMMON BUFFER AREA ON MAG TAPE TRANSFER CONTENTS OF SECOND HALF OF COMMON BUFFER AREA INTO FIRST HALF COMPUTE CURVE |,cuRvE CRUMAX, sums HAS LAST FLAG BEEN SET COMPUTE CURVE LENGTH "SUM z" FIGURE 8 PATENTEusImIIn 1609319 SIIEEI 08 0F 15 Is"suM z" GREATER THAN sHoRT LOOK AHEA LENGTH INDEX DATA POINT IDENTIFICATION COUNTERS; ALSO JX Is"suMs GREATER THAN LONG LOOK AHEAD LENGTH INDEx DATA POINT IDENTIFICATION couNTERs, ALSO JXBIL TRANSFER GoNTENTs OF ARRAYS "M" AND "cRuMAx, ONE woRD DOWN IN MEMORY SUBTRACT TRAILING SEGMENT LENGTH OF SHORT LOOK AHEAD FROM "am 2'' COMPUTE CRULAX FIGURE 9 PATENTEUSEP28I9YI 3, 09,319

SHEET 09 0F 15 HAS VELOCITY II II ADJUST FLAG KX BEEN SET Q SET"V MAxTo LOW VALUE CONSTANT SET vMAx" TO HIGH VALUE CONSTANT COMPUTE vc" 40 INTEGERIZE "vc 4| TRANSFER VALUE OF "vc" INTO TRAILING SEGMENT OF LOOK AHEAD IN COMMON BUFFER AREA SUBTRACT TRAILING SEGMENT LENGTH OF LONG LOOI'K AHEAD FROM 'suMs FIGURE l0 PATENTEDSEP28T9TT 3,509,319

SHEET 10 0F 15 HAS "LAST" FLAG BEEN SET \. COMPUTE STPLGH 45 IS TOTAL T CURVE LENGTH GREATER THAN LOOK AHEAD ENGTH (HAS 1 FLAG" COMPUTE NEW STPLGH BEEN SET LOOP THRU LOOK AHEAD TO IDENTIFY SEGMENT IN WHICH STOP MUST BE INSERTED 48 COMPUTE DUMMY sEGMENT LENGTH AND COEFFICIENTS sET NEw CONTROL WORD INSERT DUMMY SEGMENT INTO BUFFER STORAGE AREA FIGURE l l PAIEI--IIEUSEP28I9YI 3.609.319

SHEET 11 0F I5 WRITE CONTENTS OF BUFFER STORAGE AREA ON MAG TAPE STORE COORDINATES 0F LAST DATA POINT m "x PAR(?)" a "Y PARI'I)" IS STANDBY FLAG SET MOVE CARRIAGE WITH PEN UP TO ORIGIN USING STANDBY CONTROL WORD RETURN TO CALLING PROGRAM END FIGURE l2 PATENTED SEPZ 8 1971 SHEET 12UF 15 IN IT! ALIZE CON STANTS COMPUTE "A" AND "a" THE CALLING PROGRAM SGRIBE BLANK BUFFER I SET I OUT (8) TO mu? (m EQUAL To ZERO SCRIBE CURVE SET UP ACCELERATION .SEGMENT AND MOVE TO BUFFER STORAGE COMPUTE saemam LENGTH "z", "A cos", AND "B sms".

FIGURE l3 PATENTEU SEP28l97l 3,609,319

SHEET 13UF 15 62 SEGMENT LENGTH -GREATER THAN MAX.

NECESSARY TO REACH MAX. VELOCITY SET SEGMENT LENGTH I OUT(2) T0 l/2*Z 3 SET SEGMENT LENGTH I OUT (2) TO 1'! 2 B SET I FLAG'I MAX/2 GOMPUTE "I cum)" AND "I OUT(5)";

65 MOVE SEGMENT TO BUFFER STORAGE AND "I OUT (7)" FIGURE l4 PATENTEusiPzelsn I 3,509 319 sum 1n [1F 15 THIS TO BE A STANDBY CALL 67 YES SET OONTROL WORD FOR I STANDBY AND DEGELERA' TION.

SET CONTROL WORD FOR DECELERATE YES SET SEGMENT LENGTH I OUT 2= 2 l2 SET SEGMENT LENGTH I ouflz) z MAX./2

72 MOVE DECELERATION SEGMENT TO BUFFER STORAGE SET UP (SETTLE DWELL IN) SEGMENT FIGURE l5 PATENTEDSEPZBIBY! $509,319

sum 15 or 15 'MOVE DWELL SEGMENT TO BUFFER STORAGE THE CALLING PROG. SCRIBE CURVE CURVE 76\ SET UP ACCELERATION SEGMENT FOR CURVE 77\ MOVE ACCELERATION SEGMENT TO BUFFER AREA RETURN END FIGURE l6 DIGITAL PLOTTER IN WHICH PLOTTING SPEED IS OPTIMIZED IN TERMS OF A CURVE-FITTING TECHNIQUE This application is a continuation-in-part of copending application Ser. No. 725,418 filed Apr. 30, I968, now abandoned.

The present invention relates to a system for obtaining graphical plots of digital data representing lines or curves.

Interpretation of the output of a digital computer when used for solution of scientific problems has been an area of much activity in recent years. Solutions from a digital computer consjst generally of a series of numerical designations requiring detailed and time-consuming interpretation. It would be very desirable to obtain a visual display of the solution indicated by these numerical designations without the necessity for skilled interpretation.

US. Pat. No. 3,l99,l II to Jennings et al. represents a prior art system which produces a graphical plot of digital data. This system comprises essentially an incrementally movable plotting instrument controlled by a plurality of stepping motors. The motors are stepped by the digital signals representing the function to be plotted.

One disadvantage in the above-described prior art plotter is the lack of a smooth and continuous plot. This disadvantage arises from the stepwise or discontinuous nature of the plotting system. The resultant plot has the appearance of a plurality of sequential steps defining the desired configuration.

Another disadvantage of the above-described incremental plotter system is that it possesses constant speed. For many applications, such as line plotting or curve plotting where the degree of curvature is not great, greater plotting speed can be utilized enabling expeditious plotting of the particular input data.

The present plotting system eliminates the first of the above disadvantages by provision of a continuous plotter drive mechanism consisting of a hybrid servosystem which digitally compares the desired incremental position signals with the incremental present position signals to generate a digital error signal. This error signal is converted to analog form and is used to drive the x and y positioning motor.

The second disadvantage above is overcome by relating present plotter velocity to a command or desired plotter velocity. The command velocity is computed based upon the degree of curvature to be encountered and the acceleration limits of the plotter mechanism. This command velocity is thus optimized as the best plotting speed for the system capability and the requirements of the data.

It is, therefore, an object of the present invention to provide a continuous graphical plot of digital data.

Another object of the present invention is the generation of a graphical plot of digital data at a plotting speed which is optimized based on the degree of curvature of the data and the system acceleration capability.

These as well as further objects of the invention will become apparent from a reading of the following specification which refers to the accompanying drawings in which:

FIG. 1 is a diagram of a typical curve showing parameters of the curve necessary to the present invention;

FIG. 2 is a diagram of acceleration versus velocity;

FIG. 3 and 4 are block diagrams of circuitry employed in the present invention; and

FIGS. 5-16 constitute a flow chart showing the solution of certain quantities by a general purpose digital computer.

In brief, the data to be plotted is obtained in digital form from the output of a digital computer. This output data may be termed a series of data points. The degree of curvature 'between data points is generated. Optimum plotter velocity for this curvature is developed taking into account the physical limitations on speed and acceleration imposed by the mass of the plotting structure to be moved and by the torque output of the motors available to drive the plotting structure. The optimum velocity thus determined is compared with the actual plotter velocity and an accelerate or decelerate command is generated.

The segments between the data points are approximated by a third order polynomial curve-fitting technique. Third order polynomials for the data points in x and y are thus generated with distance as the independent variable. This distance between data points is also computed. The geometry of the segments is thus determined and is plotted by solving the third order polynominals at a rate determined by the accelerate or decelerate commands generated by the optimized velocity function.

THEORY OF OPERATION In FIG. 1, the digital data points to be plotted are represented by points x y x,, y x y and x y on a curve. The present invention employs an interpolation curve-fitting technique to drive the plotter pen for the curvilinear distance between data points.

The following third order polynomials define the segments of the curve between data points:

The variable 1 is a dummy variable, having the dimension of length, and is used to define the parametric equations in x and y. The coefficients (1,, b,, c, and a,,, b and c, are constant for the curve segment generated between two data points.

By definition fi wn 'wy where s actual distance along the segment; and x and y distance along the abscissa and ordinate, respectively. By defining the relationship between s, x and y as in equation (3), it is clear that:

A set of constraints are imposed on the characteristics'of the curve to define the curve between the data points. These constraints determine the value of the constants a b,, c and (1,, b and c The constraints are chosen such that:

l. The interpolating curve passes exactly through the data points such as x,, y and x y;

2. The interpolating curve has slopes at the beginning point and end point exactly equal to the slope of chords drawn between the preceding and succeeding data points;

3. The derivative dz/ds of the dummy variable 1 with respect to distance along the path s, is equal to unity at both ends of the curve (i.e. at x y and y-,) and in the middle of the curve (i.e. at z=z /2 where Z is the value of 1 at the point x y This is selected to permit dz/dl to be approximated by ds/dt and to limit the variation in curvature in the segment.

The constraints are expressed mathematically as follows:

Constraints 6 and 7 make the curve pass through data points x y (the form of equations (l) and (2) make the curve pass through x y at z=0). Constraints 8 and 9 make the slope of the curve at the beginning and end point exactly equal to the slope of the chords as described in paragraph 2, above. Constraints 10-12 make the rate of change of z with respect to s (13) o=ax=cos 71 Equations (15) and (16) are modified versions of equations (1) and 2) at z=z Equations (13) and (14) are derived as follows:

(19) sin-y,=dy/dzat Letting 0 (23) cosy,=a,, and

(24) sin-y,=a,,

Equations (17) and (18) are derived in the same manner as equations 15) and (16); however, z=z

Solution of these equations simultaneously in terms of z with the constraints imposed yields:

(25) a,=cos-y To solve for 1 consider the derivative which can be expanded to:

(3 l) (ds/dz)"'=(d,\:/dzVi-(dy/dz) Which can be expanded to Note the evaluation of the above equations at 12 Thus, ZZZW'COS (72' 'Yi]+ 2 1) 'YI' 72) +(}2"'Y1 yr 701- z" 1) *(yz-yi) (34) and z =(1/a) [(b/2)+ V (b/2) -ac] where (Yz-Yi) 2 i)( siny )a0 l( r i) +(yry1) l Computation of the angles 7 requires a preceding point and a succeeding point for each line segment. As the beginning angle of the curve 'y and the ending angle of the curve have no such points, a rule must be established for determining these angles. The rule is that angles between the tangents to the curve and the straight lines between the data points will be equal at both ends of the segment. Thus. for the beginning of the segment:

,y1yo ,y2y 2 tan tan and for the ending segment,

7n yn yn 1 n yn-yn2 u n-l. n n2 where y,,, x represent the end point of the curve.

SPEED CONTROL FIG. 2 shows a velocity acceleration saturation plot based on motor characteristics, arm weight, inertia, etc. The total velocity is controlled such that the saturation envelope of FIG. 2 will not be exceeded by the command acceleration and velocity. The total velocity and acceleration command is kept below the capability of each axis. Thus, when the segment being plotted is predominantly in x or y, the plotter will be capable of following the command and, when the segment is in other directions, there is a margin in the plotter capability over the commands.

To further simplify the control, the tangential acceleration commands are restricted to values which reserve sufficient acceleration for pulling curvature." The velocity at which the system can operate and still reserve sufficient acceleration to pull curvature" is calculated. This velocity is a function of the curvature of the segment under consideration, If the actual velocity of the system is greater than the computed velocity, the system decelerates; if vice versa, it accelerates.

A look ahead technique is utilized to determine what acceleration must be commanded to be able to maintain the required curvature at maximum velocity and still be able to decelerate and stop as required. The look ahead technique is implemented as follows:

1. A look ahead distance is developed which is of sufficient length to allow the plotter to stop from maximum speed;

2. The maximum curvature for all segments within that distance is calculated; and

3. an acceleration command is generated corresponding to that velocity required to plot the calculated maximum curvature.

The equations for developing the acceleration commands are derived as follows:

Let v actual plotter velocity v,.= command velocity a acceleration capability of plotter at v v I an x normal component of acceleration at x tangential acceleration available after reserving a, to

pull curvature at vc The normal component of acceleration is given by (40 a,.=kv2 where k is the curvature;

From the basic calculus, kxda/ds where a is the angle of inclination between the horizontal and the tangent to the curve at a point on the curve; and s is the distance along the curve.

The expression for k in terms of z is given by;

From FIG. 1, the tangent of the angles 7, or y is equal to dy/dx, the slope of the curve so that:

which reduces to: a ive @Xay dz dz dz dz ds From the first and second derivatives of equations 1 and 2 (45) kxl(byaxlaybx)+(a,c,,-a,,c,)+l/2(b,r,, b,,c,) z

dz/ Order of magnitude studies using typical values show that (dz/ds)can be assumed equal to 1 over the whole segment.

(o -22) and letting v =v yields max.

max.

Equation (48) represents the velocity v which is commanded while reserving an acceleration a for pulling the curvature kmax at the velocity v. Equation (47) represents the capability of the mechanical servosystem where 01 is the maximum acceleration (at zero" velocity) and vmax is the maximum velocity (at zero acceleration) STOP/START LENGTH Complete control of the plotter requires knowledge of the 25 start and stop length required as a function of velocity and acceleration.

The hardware is arranged so that the value vc in equation (47) can be controlled independently of the running velocity for any one curve. The magnitude of vc. therefore, defines the shape of the velocity/acceleration plot of FlG. 2.

By increasing vc, the start and stop length required to reach a velocity vmax cannot be shortened at the expense of increased accelerations. In the limit where vc is vmax, the start length is infinite, and the accelerations are minimum. The start length is equal to the stop length in all cases and is expressed as (49) 2 :1; [m-v-l-mln v max c v v starting velocity at :xO v x velocity at distance 2; and

Io- 2 9925mm-.. H w For accelerational, v0x0; vxvmaxwc x constant; and the sign of V /o is taken positive. For deceleration; v =v,,,,,,; v=0; v the same constant; and the sign of vmax/a is taken negative.

The above equation is derived from equation (47) by integrating twice with respect to z, and noting that (50 oz=(dv/dt)=(dv/dz)dz(dt)=(dvdz) (v); where a ac- 50 celeration.

COMPUTER OPERATIONS A portion of the above-defined equations are solved on a general purpose digital computer such as the IBM 360. Figures 5-12 show the flow chart for generating the quantities ax. ay, bx, b,,, cxx, cy, z and vc for the curve mode.

The following tables describe the program to develop the quantities.

TABLE 1 SUBPROGRAM ORGANIZATION TABLE l-Continued Name Language Function Data management: convert l'ull word to hall'word and place in a specified address in common storage Data management; shill l00 words of specified storage down one word, erasing first word as a result Data management; retrieve 12 words of specified common storage. convert to 23 full words. and move to specified storage in FORTRAN program. Move specified number of words up 12 words in common storage starting at specified location. lnscrt new control word velocity hall word into specified number of segments starting with specified address in common storage.

l/O Write specified number of full words into magnetic tape starting with first word of common storage.

of common storage.

Computational: Uses INSRP and OUT to write tape to more carriage with pen up to beginning of plot.

VELC Assembly TRANS Assembly GETCO Assembly OUT Assembly ORIG FORTRAN Control Word Format Code Control Description Decimal Hexadecimal Start of word (Control word less specific commands) )Ol I20 000DC000 New Curve or line Add 8192 Add 00002000 Standby (last word) Add 4096 Add 0000l000 Pen Up it yes. add If yes. add

1024 00000400 Line mode "yes. add If yes. add

256 00000100 End of curve or line If yes. add 2 if yes. add

(decelerate) 00000002 Examples: Curve mode. pen down. not first segment Decimal Hexadecimal 901 I20 ooobCooo Line mode. pea up. first segment (new line) Decimal Hexadecimal 910592 000DE500 TABLE 11 Variable Description initial value X..... Array of X coordinates" Front calling program. Y Array of Y coordinates Do. XPAR Array of control para1nctcrs-. Do. do Do. Number of array points. Do. ZA. Array of segment length in None.

look ahead. CRVMAX. Array of curvatures in look Do.

ahead. t BUFFER... Array of plotter commands Entire area set to in common storage. blanks. ASQW. Function for extracting vector None.

length from delta X and delta Y. J First of the three points 1.

under computation at any one time. 1 Second of those points 2. M- Third of those points 3. L.. i. Number oi segments in long 1.

length look ahead. LX Number of scgments in com- 1.

mon storage into which a velocity value has been inserted.

KX Both flag and counter for 0.

maximum velocity adjust ment in section of discoutinuous velocity.

JX. Number of segments in long 1.

length look ahead.

J SW Flag to indicate standby at 1 (no standby).

end of plot.

CXO X coordinate of last point of None.

previous curve.

CYO Y coordinate of last point of D0.

previous curvo.

TABLE II-Continued Variable Description Initial value IBUF Subscript variable of 1.

BUFFE R.

KOUNTZ. KOUNT-l NO]3 Maximum size of record 990.

(words) to be written onto magnetic tape at any one time.

IFLIP Flag used for dash line mode (solid line portion of (1 is for blank portion of ash lin dash line).

VMAY. Maximum commanded 16.

velocity.

AMAX.. Maximum normal acceleration. 24.

LAST..... Flag indication last point in 0.

plot has been encountered.

OLSCRV. Flag indicating a closed curve. 0.

IFLAG. Flag indicating Whether total 0.

length of curve is greater than look ahead.

Length of dummy request.... 0. Addresses of storage into which subprogramGETCO transfers coefiicient from common storage. (Order of 0. appearance in Fortran is important and must be as shown). Sum of lengths of segments 0.

included in long length look ahead at any one time.

SUMZ. Sum of lengths of all segments 0.

included in short length look ahead at any one time.

IOUT2.. Segment length IOU'I4... AX toetlicient.

IOUIfi AY eoellicient IOUTG BX coefficient Do.

lOUT7.. BY r-oellicient- IOUT8.. (3X coeliicicnt D0.

IOUIU ("Y coeilicient. Do.

IOUT10.. X position at beginning of Do.

segment.

U'IlL. Y position at beginning of Do.

Segment.

DESCRIPTION OF FLOW CHARTS The numbers preceding each paragraph refer to the block number of the flow chart. Curve-FORTRAN l. The coordinate positions of each data point in the given .array are doubled. This is done so that the coefficient of the third order parametric equations (1) and (2) can be halved after they are computed and before they are written onto magnetic tape. This prevents overflow from occuring in the coefficient register of the final DDA integrators. These would be the AX and AY" The scale factor is also introduced at this point. Note that the scale factor is a divider so that a factor of 4 would result in a plot one-fourth the size of the plot described by the given data array.

2. The parameter XPAR" (6 is a flagto indicate whether the pen is starting from the origin (XPAR (6)=0) or from the end of some previous plot (XPAR (6)=l 3. CALL ORIG subprogram to write plotter instructions on magnetic tape that will move the carriage with pen up to the beginning of plot. The previous block identified the current position of the carriage. XPAR (7) and YPAR (7) serve to transfer the coordinate of the end of the previous plot if any. XPAR (3) and YPAR (3) are the origin coordinates of the current plot. The coordinate of the last point of the previous plot are given by:

4. Table 11 lists the variables giving name, description, and initial value of each. Those variables which require setting to an initial value are so set at this point in the program.

5. The 1 counter is tested for equality to l.

6. The first and last coordinate values are tested for equality to each other. If the first point has the same position as the last, the CLSCRV" flag is set.

7. The l counter is tested for equality to KOUNT.

8. The following equations are executed:

(53) DEL X2xX(M)-X(J) (54) DEL y2xY(M) Y(J) (55) ASQZX SQUARE ROOT (DEL X2) (DEL X2) (DEL Y2) (DEL Y2) (56) ACOSZX DEL X2/ASQ2 (57) ASINZX DEL Y2/ASQ2 9. Set last 1. Test LX for equality to l.-] flag 21 is LX 1. This indicates that total curve length is less than look ahead distance.

10. For a closed curve, the following equations are executed:

(58) DEL Xl x X(l) X(KOUNZ) (59) DEL Ylx Y(T) Y(KOUNZ) (60) DEL X2 x X(M) X(J) (61) DEL Y2 x Y(M) Y(2) (62) AXQl x SQUARE ROOT (DEL Xl DEL X1 2 DEL Yl z DEL Yl) (63) ASQZ x SQUARE ROOT (DEL X2* DEL X2 2 DEL Y2 z DEL Y2) (64) ACOS l x DEL Xl/ASQ] (65) ASIN l x DEL Yl/ASQl (66) ACOS 2 x DEL X2/ASQ2 (67) ASIN 2 x DEL Y2/ASQ2 If the curve is not closed, the following equations are executcd:

(68) DELXl x X(I)X(J) (69) DEL Yl x Y(I)(J) (70) DEL X2 x X(M) -X(J) (71) DEL Y2 x Y(M) Y(J) (72) ASQl XSQUARE ROOT (DEL Xl* DEL X1 2 DEL Yl* DEL Y1) (73) ASQZ x SQUARE ROOT (DEL X2* DEL X2 2 DEL Y2* DEL Y2) (74) ACOSl x DEL Xl/ASQl (75) ASINl x DEL Yl/ASQl (76) ACOS2 x DEL X2lASQ2 (77) ASIN2 x DEL Y2/ASQ2 (78)ADCOSl XACOSl* ACOSI I ASINI" ASlNl (79) ADSINl x 2 z ASIN2 z ACOSl (80) ASINl xADSINl zACOSZ I ADCOSl zASINZ (81 ACOSl x ADCOSl 2 ACOS2 z ADSINI z ASINZ 11. The segment length 22 and the coefficient AX, A Y, BX, BY, CX, CY can all be computed from equations 12A through 12F, 15 and 16 with the following substitutions:

(82) X2 1 X] x DELXl (83) Y2] Y1 xDELYl (84) COS 7 ==AC0S2 (85) COS y,=ACOSl (86) Sin 7, ASIN2 (87) Sin 'y, =ASINl 12. Each of the coefficients are tested for absolute magnitude. If the BX or BY coefficients exceed 2 or the CX or CY coefficient exceed 4, the branch to a recomputc sequence is affected.

13. & 14. The following equations are executed: (88) AX x AX/2 (89) A Y x A Y/2 (90) BX x BX/4 (91) BY x BY/4 (92) CX x CX/8 (93) CY x CY/8 (94) Z x Z* 2 A study of equations l) and (2) will show that the above alteration of the coefficient does not change the geometry of the curve. It does, however, reduce the magnitude of each coefficient. This sequence is repeated until the coefficient are less in magnitude than the above specified limits. These limits are set by the nature of the DDA integrator and the size of their registers. Also too small a value in the coefficient register of either the B or C" integrators will result in appreciable error after a long integration length. This sequence will have the undesirable effect of halving, quartering, etc., the velocity of the plotter upon encounter with this segment. To minimize 

1. A system for plotting digital data points comprising: means to compute optimum plotter velocity, distance between the data points, and the coefficients of polynomial equations for the data points; means connected to said computing means to receive said optimum velocity and to compare said optimum velocity with actual velocity; means connected to said comparison means to generate an accelerate or decelerate command signal; means to receive said accelerate or decelerate signal for incrementing or decrementing said actual velocity; means connected to receive said accelerate or decelerate signal to produce an incremental distance signal at a rate proportional to said actual velocity; means connected to receive said distance signal and to decrement said distance signal by said incremental distance signal; means connected to said computing means to receive said coefficients and said incremental distance signal and solve said polynomial equations at a rate proportional to the rate of said incremental distance signals for the time required to decrement said distance by said incremental distance: and, means to plot said solutions.
 2. A plotting system for producing a continuous plot of data points in the form of digital signals employing an interpolating Curve-fitting technique for generating the configuration of segments between data points, said system comprising: a variable velocity plotting mechanism for plotting said data points and said segments; computer means for producing first output signals representing coefficients of an interpolating curve for defining said segment, second output signals representing the velocity at which said segment is to be plotted, and third output signals representing the length of said segment to be plotted; means connected to said computing means and to said plotting mechanism for producing a difference signal representing the sign and the magnitude of the difference between the velocity of said plotting mechanism and said second output signals; means connected to receive said difference signal and said third output signal for producing a binary control signal whose pulse repetition rate is a function of said difference signal; digital differential analyzer means connected to receive said first output signal, and said control signal for generating an output signal representing said segment at a rate proportional to said repetition rate; and means connecting said output signal to said plotting mechanism thereby producing a graphical plot of said segment.
 3. A system for providing graphical plots of information points having the form of digital signals where curve fitting is employed to define the configuration of the portions of the plot between said points comprising: a digital computer for producing, with respect to a predetermined portion of said plot, first curve-fitting output signals for defining said predetermined portion, a second output signal representing the speed at which said portion is to be plotted, and a third output signal representing the length of said portion; comparison means connected to said computing means for receiving said second output signal for generating an incremental position signal having a pulse repetition rate functionally related to said second output signal; control means connected to said computing means for receiving said third output signal, and to said comparison means for receiving said incremental position signal to control the production of said position signal as a function of said third output signal; digital differential analyzer means connected to receive said first output signals and said incremental position signal for producing output signals defining said portion at a pulse repetition rate proportional to the pulse repetition rate of said position signal; and a plotting mechanism connected to said digital differential analyzer means for plotting said output signals.
 4. A system for plotting digital data points employing curve fitting to plot segments between data points comprising: means to compute, for a given segment, optimum plotter velocity, distance between the data points, and the coefficients of polynomial equations for said curve fit; means connected to said computing means to receive said optimum velocity and to compare said optimum velocity with actual velocity; means connected to said comparison means to generate an accelerate or decelerate command signal as a function of said comparison; means to receive said accelerate or decelerate command signal for respectively incrementing or decrementing said actual velocity; means connected to receive said accelerate or decelerate signal to produce an incremental distance signal having a pulse repetition rate proportional to said actual velocity; means connected to said computer means to receive said distance signal for decrementing said distance signal by said incremental distance signal; means connected to receive said coefficients for said curve fit and said incremental distance signal to solve said polynomial equations at a rate proportional to the pulse repetition rate of said incremental distance signal for the time required to decrement said distance by said incremental distance; and means to plot said solutions.
 5. The system of claim 4 wherein said means for plotting said solutions includes a plotter mechanism and a hybrid servosystem for driving said mechanism, and said coefficient receiving means includes a plurality of digital differential analyzers connected in series, the output of said differential analyzers being connected to said hybrid servosystem.
 6. The system of claim 5 wherein said plotter mechanism may be driven at a range of speeds and wherein said rate of solutions determines a speed within said range.
 7. A plotting system for producing a graphical plot of data points in the form of digital signals, by employing an interpolating curve-fitting technique for generating the configuration of segments between data points, said system comprising: computer means for producing first output signals representing coefficients of an interpolating curve for defining said segments, second output signals representing the velocity at which said segments are to be plotted, and third output signals representing the length of said segments to be plotted; means connected to said computing means and to said plotting means for producing a difference signal representing the sign and the magnitude of the difference between the actual velocity of said plotting means and said second output signals; means connected to receive said difference signal and said third output signals for producing, for each segment, a control signal whose pulse repetition rate is a function of said difference signal; means connected to receive said first output signal and said control signals for generating, for each segment, a fourth signal representing said segment at a rate proportional to said repetition rate; and variable velocity plotting means for producing a graphical plot of said fourth signal.
 8. The plotting system of claim 7 further including a hybrid servosystem connected to said plotting means for moving said plotting means in a continuous manner in response to said fourth signal.
 9. The plotting system of claim 7 wherein said fourth signal generating means includes a plurality of serially connected digital differential analyzers.
 10. The method of producing a continuous graphical plot of data points in the form of discontinuous electrical signals employing interpolating curve fitting for generating the configuration of the plot between data points comprising the steps of utilizing computer means for generating from said data points first discontinuous electrical signals representing the coefficients of an interpolating curve for defining the configuration of a plot between said data points; second discontinuous electrical signals representing the velocity at which said configuration is to be plotted; and third discontinuous electrical signals representing the length of said configuration; generating a fourth discontinuous electrical signal representing the difference between the actual velocity of a plotting means and said second electrical signal, generating a fifth discontinuous electrical signal having a pulse repetition rate proportional to the magnitude of said fourth electrical signal; utilizing said first electrical and said fifth signal to generate a further electrical signal representing said configuration, at a rate proportional to said repetition rate; and plotting said further electrical signal on said plotting means. 